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[Histology](https://durgical.com/en/histology/) · [Urinary system](https://durgical.com/en/histology/#a-urinary)

# Kidney: cortex, medulla and the nephron

- Organ

The nephron is the unit that filters blood and makes urine; each segment has its own epithelium, which explains its job.

Updated 2026-11-01

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- The proximal convoluted tubule is the longest segment and the most vulnerable to ischemia and toxins: eosinophilic cuboidal epithelium with a brush border.
- The thick ascending limb is impermeable to water and reabsorbs sodium through the Na-K-2Cl cotransporter, the target of loop diuretics.
- The distal convoluted tubule reabsorbs sodium through the Na-Cl cotransporter, the target of thiazides.
- In the collecting duct, principal cells answer aldosterone and ADH (aquaporin 2); intercalated cells handle acid-base balance.
- Juxtamedullary nephrons have long loops that dip into the medulla and build the gradient that concentrates urine.

## What it is

The kidney has a granular cortex, holding the renal corpuscles and convoluted tubules, and a medulla of pyramids with loops of Henle and collecting ducts. Each kidney holds about a million nephrons. Knowing the epithelium of each segment tells you where diuretics act and which segment is hurt in acute kidney injury.

## Structure

- Cortex: round renal corpuscles, many pink proximal tubule profiles with a fuzzy lumen, and fewer, paler distal tubules with a wide lumen.
- Medullary rays: bundles of straight tubules and collecting ducts running up from the medulla into the cortex.
- Medulla: pyramids with thin limbs of squamous epithelium, thick limbs and collecting ducts of cuboidal to columnar cells with sharp cell borders.
- Papilla: collecting ducts merge into papillary ducts that empty into the minor calyces.

## Function

- Filters plasma in the glomerulus and reabsorbs most of the filtrate along the tubule.
- Regulates volume, sodium, potassium and pH, and concentrates or dilutes urine under ADH.
- Also works as an endocrine gland: renin, erythropoietin and active vitamin D.

## Nephron segments: epithelium and main job

| Segment | Epithelium | Main job |
| --- | --- | --- |
| Renal corpuscle | Simple squamous parietal layer of Bowman's capsule; visceral layer of podocytes | Filtration of plasma |
| [Proximal convoluted tubule](https://durgical.com/en/conditions/acute-kidney-injury/) | Simple cuboidal with brush border, eosinophilic cytoplasm and many mitochondria | Reabsorbs most sodium and water and nearly all glucose and amino acids |
| Loop of Henle, thin limbs | Simple squamous | The descending limb lets water out into the hypertonic medulla |
| Thick ascending limb | Simple cuboidal without brush border | Reabsorbs Na, K and Cl without water; dilutes the filtrate; makes uromodulin |
| Distal convoluted tubule | Simple cuboidal, few microvilli, lower and paler cells | Reabsorbs sodium and calcium; the macula densa senses arriving chloride |
| Collecting tubule and duct | Cuboidal to columnar with sharp borders: principal and intercalated cells | Final adjustment of sodium, potassium, acid and water under aldosterone and ADH |

## Clinical notes

- Acute tubular necrosis: the proximal tubule loses its brush border and its cells slough; muddy brown granular casts appear in the urine.
- Chronic kidney disease: the microscope shows sclerosed glomeruli, tubular atrophy and interstitial fibrosis.
- Nephrogenic diabetes insipidus: the collecting duct does not respond to ADH and urine stays dilute.

## Memory aid

Think of the nephron as a kitchen: the proximal tubule keeps almost everything useful, the loop salts the medulla, the distal tubule fine-tunes the seasoning and the collecting duct decides how much water goes on the plate.

## Test yourself

Think of the answer, then tap the question to see it.

## Which nephron segment has a brush border and is the most damaged in acute tubular necrosis?

The proximal convoluted tubule: cuboidal epithelium with long microvilli and many mitochondria, which make it very sensitive to ischemia and toxins.

## Why does the thick ascending limb dilute the filtrate?

It reabsorbs sodium, potassium and chloride through the Na-K-2Cl cotransporter but is impermeable to water, so solute leaves without water.

## Which collecting duct cells insert aquaporin 2 when ADH acts?

The principal cells.

## Which type of nephron builds the medullary gradient that concentrates urine?

The juxtamedullary nephron, whose long loop of Henle reaches deep into the medulla.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/histology/kidney-cortex-medulla-and-the-nephron/)

## See it in 3D

The structures of Durgical's 3D anatomy where it is found.

- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related

- [Glomerulus and the filtration barrier: podocytes, mesangium, JGA](https://durgical.com/en/histology/glomerulus-and-the-filtration-barrier-podocytes-mesangium-jga/)
- [Ureter, bladder and urethra: urothelium](https://durgical.com/en/histology/ureter-bladder-and-urethra-urothelium/)
- [Simple, stratified, pseudostratified and transitional epithelium](https://durgical.com/en/histology/simple-stratified-pseudostratified-and-transitional-epithelium/)
- [Cilia, microvilli and stereocilia](https://durgical.com/en/histology/cilia-microvilli-and-stereocilia/)

## Related conditions

- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Autosomal dominant polycystic kidney disease](https://durgical.com/en/conditions/autosomal-dominant-polycystic-kidney-disease/)

## In Pharmacology

- [Furosemide](https://durgical.com/en/pharmacology/furosemide/)
- [Hydrochlorothiazide](https://durgical.com/en/pharmacology/hydrochlorothiazide/)
- [Spironolactone](https://durgical.com/en/pharmacology/spironolactone/)

[← All of histology](https://durgical.com/en/histology/)

Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: [25.4 Microscopic Anatomy of the Kidney](https://openstax.org/books/anatomy-and-physiology-2e/pages/25-4-microscopic-anatomy-of-the-kidney) (OpenStax, Rice University, 2022); [25.3 Gross Anatomy of the Kidney](https://openstax.org/books/anatomy-and-physiology-2e/pages/25-3-gross-anatomy-of-the-kidney) (OpenStax, Rice University, 2022); [Kidneys](https://training.seer.cancer.gov/anatomy/urinary/components/kidney.html) (National Cancer Institute, SEER Training Modules, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/histology/kidney-cortex-medulla-and-the-nephron/